A typical digital optical communications system includes a transmitter, an optical channel (e.g., optical fibre), and a receiver. Light, generated and modulated by the transmitter, travels through the optical channel and is detected by the receiver. The receiver demodulates the received light to recover the original transmitted data signal.
The quality of a data transmission system is expressed as a ratio of the number of incorrectly received bits to the number of corrected received bits, also known as the bit error rate (BER). Modern data transmission systems typically require a BER of less than 10−12 to be considered commercially viable.
Optical transmission systems are inherently lossy. Physical phenomena degrade the transmitted signal, limiting its integrity at the receiver, causing the incorrect detection of one or more bits. Some of these phenomena are statistical in nature, while others are bulk effects. Generally, the higher the data transmission rate, the higher the BER—other factors being held constant. Many techniques of improving the BER while increasing the data rate are known.
One well-known technique is forward error correction (FEC). One example of an FEC coding scheme used ‘block codes’. This technique operates by dividing the transmitted data into blocks. Additional bits, known as check bits, are generated by processing the bits of the block and added to the block before transmission. The receiver detects the bits of the augmented block, processing the received original data bits to generate a local version of the check bits. Differences in the generated and received check bits indicate and permit correction of errors in the transmitted data. As the ratio of check bits to original data bits increases, the ability of the system to detect and correct single- and multiple-bit errors increases, decreasing the overall system BER. Check bit generation schemes are well known in the art, examples of some techniques can be found in ‘A common sense approach to the theory of error correction codes’, by Benjamin Arazi (ISBN 0-262-01098-4), incorporated herein by reference.
Systems with FEC are said to have two bit error rates. The first is the error rate of the data transmission system before the corrective effects of FEC, which is known as the raw bit error rate (raw BER). The second is the error rate of the data transmission system after the corrective effects of FEC, which is known as the system bit error rate (system BER) or, sometimes, simply as the bit error rate (BER). The difference between the raw BER and the system BER is determined by the effectiveness of the actual error correction code. Commonly used FEC coding techniques provide approximately a 108 reduction in BER with approximately a 7% increase in transmitted data rate requirements. In a typical system, this translates into the tolerance of up to a 10−4 raw BER for the physical transmission system while the system BER is still below the 10−12 market requirement.
Phenomenon in an optical transmission system occur that degrade the quality of the transmitted signal, reducing the signal to noise ratio (SNR) at the receiver and leading to an increase in the BER. Techniques have been developed to compensate for these effects including: chromatic dispersion compensation, polarization mode dispersion compensation, optical filtering, electrical filtering, decision threshold adjustment, and optical gain adjustment. Each of these compensation techniques counteracts or ameliorates one or more of the phenomena to increase the SNR, decreasing the BER.
Higher transmitted bit rates may require additional compensation as compared to lower bit rate systems. For example, at 2.5 Gigabits/second (Gbs), optical signals are relatively unaffected by chromatic dispersion for distances of up to several hundred kilometers. However at 10 Gbs (i.e., four times the data rate), chromatic dispersion is 16 times as severe, requiring dispersion management via dispersion compensating fibre, low dispersion fibre, active dispersion compensation or other methods.
Similarly, increasing the data rate to 40 Gbs increases the chromatic dispersion effects by an additional factor of 16 so that dispersion compensation has to be accurate within tens of picoseconds per nanometer.
Some of the compensation techniques require tuning or adjustment to operate. Compensation techniques that have been applied to lower bit-rate systems have required only static tuning. For these systems, tuning is performed once during installation by skilled personnel and may not need to be repeated thereafter.
As described above, higher data rates place an additional burden on the existing compensation techniques and, in some cases, require additional techniques to ameliorate phenomena that were previously inconsequential. Some of the phenomena that are being compensated for are inherently time varying in nature. Previous, static, tuning strategies are often no longer sufficient; dynamic self-tuning strategies are required.
Even at higher data rates, some of the compensation techniques can continue to use static tuning strategies if the associated phenomenon has the appropriate characteristics. However, economic considerations make a self-tuning system desirable as it avoids the need for skilled personnel to perform the tuning process.
Some mechanisms are already known in the art to provide self-tuning. U.S. Pat. No. 6,081,360, herein included by reference, discloses a system in which dispersion compensation is automatically adjusted by monitoring the power level of a particular frequency component of an optical signal sent from the transmitter to the receiver. However, the power level of an optical signal is at best an indirect indicator of the proper setting for a dispersion compensator.
A goal of the invention is to provide a self-tuning optical communications system using direct tuning indications.